Permeation mechanisms through MFI zeolite and molecular sieve membranes.

摘要

The permeation of several alkane and aromatic mixtures was studied through various MFI zeolite and molecular sieve membranes. The membranes were hydrothermally synthesized on tubular alumina or stainless steel mesoporous supports by a variety of techniques. Non-zeolite pores separated mixtures of n-hexane/2,2 dimethylbutane and n-butane/i-butane by preferential adsorption of the linear isomer. The branched isomer did not affect the permeation of the linear isomer. These pores are larger than zeolite pores but are apparently in the nanometer size range. Smaller pores were packed more effectively, and remained packed to higher temperatures. The maximum hexane selectivity was 650. Experimental configuration affected the permeation by changing the coverage gradients and extent of pore packing. The maximum butane selectivity was 140 for a pressure drop configuration, but only 40 for a sweep gas configuration. As a single gas, i-butane permeated faster with the pressure drop, apparently because of a larger coverage gradient. With the sweep gas, n-butane permeated faster.; Zeolite pores separated alkanes by a difference in adsorption coverage and diffusion rates. Single gas and mixture permeances for the butanes through these membranes were similar. Ideal selectivities increase in the order pressure drop sweep gas vacuum (maximum of 120), whereas separation selectivities increase in the order vacuum pressure drop sweep gas (maximum of 30). These results indicate the difficulty of comparing zeolite membranes characterized by different methods.; Several membrane types were used for aromatic separations. Membranes made from SAPO-5, SAPO-11 and mordenite all exhibited single file diffusion, resulting in ideal selectivities greater than one but no separation selectivities. Surface diffusion and activated gaseous transport controlled transport through the MFI (silicalite-1, ZSM-5, and boron substituted ZSM-5) membranes. The highest pxylene/o-xylene selectivities (130 ideal, 60 separation) were obtained for a boron substituted ZSM-5 membrane. Zeolite pores preferentially permeated p-xylene, and took as long as 8 h to reach steady state. Non-zeolite pores preferentially permeated o-xylene after much shorter breakthrough times. Higher pressures of p-xylene distorted the membrane framework, resulting in increased o-xylene permeation and reduced selectivity. After reaching steady state, p-xylene flux was stable, but oxylene fouled the non-zeolite pores.